arXiv subjects

Ambarish Ghosh

Publications and source records attributed to Ambarish Ghosh.

8 recordsLinked to original sources

arXiv2024

Dynamics of colloidal rods rotating in viscoelastic media

We experimentally investigate the in-plane rotational motion of ferromagnetic colloidal rods immersed in viscoelastic media and subjected to a rotating magnetic field. Unexpectedly, we observe significant angular velocity even at field frequencies an order of magnitude exceeding the step-out frequency, a regime where rods typically cease rotating in Newtonian fluids. This anomalous behavior arises from the interplay between the rapid rod actuation driven by the external field and the slower microstructural relaxation of the viscoelastic fluid. A minimal model incorporating memory effects quantitatively captures our experimental findings. Our study demonstrate a rather general case of microrheological probe dynamics in viscoelastic media where the behavior beyond step-out frequency depends strongly on the rheological parameters medium. Additionally, we derive an analytical expression for the rod orientation in the high-frequency limit, providing a potential method for extracting rheological parameters.

cond-mat.soft↗

arXiv2022

Using Thermal Ratchet Mechanism to Achieve Net Motility in Magnetic Microswimmers

Thermal ratchets can extract useful work from random fluctuations. This is common in the molecular scale, such as motor proteins, and has also been used to achieve directional transport in microfluidic devices. In this work, we use the ratchet principle to induce net motility in an externally powered magnetic colloid, which otherwise shows reciprocal (back and forth) motion. The experimental system is based on ferromagnetic micro helices driven by oscillating magnetic fields, where the reciprocal symmetry is broken through asymmetric actuation timescales. The swimmers show net motility with an enhanced diffusivity, in agreement with the numerical calculations. This new class of microscale, magnetically powered, active colloids can provide a promising experimental platform to simulate diverse active matter phenomena in the natural world.

cond-mat.soft↗

arXiv2022

Design of acoustic diffraction plates for manipulating ultrasound in liquid Helium

Many experiments in liquid Helium, such as the optical imaging of exploding electron bubbles, which enables research on individual particles under applied conditions, involve the usage of ultrasound generated by piezoelectric transducers. Previous studies either use planar transducers, which limits the maximum sound intensity and the spatial resolution, or curved transducers, which only allow observations at fixed foci and make it difficult to apply uniform electric fields. In this paper, we introduce the usage of acoustic diffraction plates in liquid Helium to amplify ultrasonic pressure oscillations at an arbitrary set of primary foci coupled with large counts of secondary foci, all of which can be freely moved around by changing the ultrasound frequency. The frequency dependence also allows us to generate controlled Faraday instabilities at the surface, which enables the generation of multi-electron bubbles with desired parameters.

cond-mat.soft↗

arXiv2016

Helical Nanomachines for Fast Mechanical Mapping of Heterogeneous Environments

Artificial micro and nano machines have been envisioned and demonstrated as potential candidates for variety of applications, ranging from targeted drug or gene delivery, cell manipulation, environmental sensing and many more. Here, we demonstrate the application of helical nanomachines that can measure and map the local rheological properties of a complex heterogeneous environment. The position of the helical nanomachine was controlled precisely using magnetic fields, while the instantaneous orientation provided an estimation of the viscosity of the surrounding medium with high spatial and temporal accuracy. Apart from providing viscosity estimates in purely viscous and viscoelastic media with shear rate independent viscosity (Boger fluids), their motion was also found to be extremely sensitive to fluid elasticity. Taken together we report a promising new technique of mapping the rheological properties of a complex fluidic environment by helical nanomachines with high spatial and temporal resolutions, a functionality that goes beyond the capabilities of existing passive and active microrheological methods.

physics.chem-ph↗

arXiv2012

Dynamical Configurations and Bistability of Helical Nanostructures under External Torque

We study the motion of a ferromagnetic helical nanostructure under the action of a rotating magnetic field. A variety of dynamical configurations were observed that depended strongly on the direction of magnetization and the geometrical parameters, which were also confirmed by a theoretical model, based on the dynamics of a rigid body under Stokes flow. Although motion at low Reynolds numbers is typically deterministic, under certain experimental conditions, the nanostructures showed a surprising bistable behavior, such that the dynamics switched randomly between two configurations, possibly induced by thermal fluctuations. The experimental observations and the theoretical results presented in this letter are general enough to be applicable to any system of ellipsoidal symmetry under external force or torque.

cond-mat.soft↗

arXiv2007

Janus particles with coupled electric and magnetic moments make a disordered magneto-electric medium

We demonstrate that by combining permanent electric and magnetic moments in particles, it is possible to realize a new type of medium that allows for a cross-correlation between electric and magnetic properties of matter, known as magnetoelectric coupling. Magnetoelectric materials have so far been restricted to systems that exhibit long-range order in their electric and magnetic moments. Here, we show that a room-temperature, switchable magnetoelectric can be realized that is naturally disordered. The building blocks are Tellegen particles that orient in either an electric or a magnetic field.

cond-mat.mtrl-sci↗

arXiv2007

Observation of the Faraday effect via beam deflection in a longitudinal magnetic field

We report the observation of the magnetic field induced circular differential deflection of light at the interface of a Faraday medium. The difference in the angles of refraction or reflection between the two circular polarization components is a function of the magnetic field strength and the Verdet constant. The reported phenomena permit the observation of the Faraday effect not via polarization rotation in transmission, but via changes in the propagation direction in refraction or in reflection. An unpolarized light beam is predicted to split into its two circular polarization components. The light deflection arises within a few wavelengths at the interface and is therefore independent of pathlength.

physics.optics↗

arXiv2006

Chiral molecules split light: Reflection and refraction in a chiral liquid

A light beam changes direction as it enters a liquid at an angle from another medium, such as air. Should the liquid contain molecules that lack mirror symmetry, then it has been predicted by Fresnel that the light beam will not only change direction, but will actually split into two separate beams with a small difference in the respective angles of refraction. Here we report the observation of this phenomenon. We also demonstrate that the angle of reflection does not equal the angle of incidence in a chiral medium. Unlike conventional optical rotation, which depends on the path-length through the sample, the reported reflection and refraction phenomena arise within a few wavelengths at the interface and thereby suggest a new approach to polarimetry that can be used in microfluidic volumes.

physics.optics↗